[PATCH v4 2/2] tracing: fgraph: use scoped_guard() for the task list walk

[Vineet Gupta <vineet.gupta@linux.dev>]

alloc_retstack_tasklist() open-codes rcu_read_lock() and unwinds it
through an unlock label. Use scoped_guard(rcu) instead and let the
compiler emit the unlock on every exit path.

The -EAGAIN case now jumps straight to the free label rather than to
unlock; leaving the guard's scope releases the RCU read lock on the
way, so the unlock label has no remaining users and is removed.

No functional change: objdump of fgraph.o shows identical call targets
and counts, including the three __rcu_read_unlock() sites the compiler
emits for the exits from the read-side section. A PROVE_RCU +
DEBUG_ATOMIC_SLEEP build with the -EAGAIN path forced several thousand
times is clean.

The preceding patch deliberately left this alone so that it stays
straightforward to backport. This is the follow-up.

Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Vineet Gupta <vineet.gupta@linux.dev>
Tested-by: Vineet Gupta <vineet.gupta@linux.dev>
---
 kernel/trace/fgraph.c | 39 +++++++++++++++++++--------------------
 1 file changed, 19 insertions(+), 20 deletions(-)

diff --git a/kernel/trace/fgraph.c b/kernel/trace/fgraph.c
index 4c74f1a15ad4..bbf6e4e2da2d 100644
--- a/kernel/trace/fgraph.c
+++ b/kernel/trace/fgraph.c
@@ -1057,32 +1057,31 @@ static int alloc_retstack_tasklist(unsigned long **ret_stack_list)
 		}
 	}
 
-	rcu_read_lock();
-	for_each_process_thread(g, t) {
-		unsigned long *rs;
+	scoped_guard (rcu) {
+		for_each_process_thread(g, t) {
+			unsigned long *rs;
 
-		if (t->ret_stack)
-			continue;
+			if (t->ret_stack)
+				continue;
 
-		rs = kmem_cache_alloc(fgraph_stack_cachep, GFP_NOWAIT);
-		if (!rs) {
-			if (start == end) {
-				ret = -EAGAIN;
-				goto unlock;
+			rs = kmem_cache_alloc(fgraph_stack_cachep, GFP_NOWAIT);
+			if (!rs) {
+				if (start == end) {
+					ret = -EAGAIN;
+					goto free;
+				}
+				rs = ret_stack_list[start++];
 			}
-			rs = ret_stack_list[start++];
-		}
 
-		atomic_set(&t->trace_overrun, 0);
-		ret_stack_init_task_vars(rs);
-		t->curr_ret_stack = 0;
-		t->curr_ret_depth = -1;
-		/* Make sure the tasks see the 0 first: */
-		smp_store_release(&t->ret_stack, rs);
+			atomic_set(&t->trace_overrun, 0);
+			ret_stack_init_task_vars(rs);
+			t->curr_ret_stack = 0;
+			t->curr_ret_depth = -1;
+			/* Make sure the tasks see the 0 first: */
+			smp_store_release(&t->ret_stack, rs);
+		}
 	}
 
-unlock:
-	rcu_read_unlock();
 free:
 	for (i = start; i < end; i++)
 		kmem_cache_free(fgraph_stack_cachep, ret_stack_list[i]);
-- 
2.53.0-Meta